This application claims the priority of German Patent Document 102 06 767.8, filed on Feb. 19, 2002, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for determining the atmospheric pressure on the basis of the intake pressure measured downstream of an air filter in an intake line of an internal combustion engine and the air mass flow rate measured downstream of the air filter and of the intake air temperature.
The increasing demands made in terms of power, exhaust emissions and comfort in modern internal combustion engines can be met only by using an engine electronic controller. It senses the operating parameters of the internal combustion engine, for example the rotational speed, temperatures, pressures, and determines from them optimum setting values for the engine-actuating variables, for example start of injection, duration of injection, charging pressure and the exhaust gas feedback rate. In order to measure the operating parameters, sensors are used, for example atmospheric pressure sensors, intake pressure sensors, intake air temperature sensors or air mass flow rate meters. Sometimes it is also possible to derive operating parameters from other measured variables and thus save the costs for sensors.
German Patent Document DE 197 10 981 A1 discloses a method of the generic type for determining the degree of contamination of an air filter. It discloses two alternatives. On the one hand it is proposed to measure the pressure prevailing downstream of the air filter in the intake tract of an internal combustion engine by means of a sensor. In addition, the ambient pressure is to be sensed by means of a sensor, for example for the air conditioning system, which is arranged outside the intake tract, and the degree of contamination of the air filter is subsequently measured from the pressure difference. It is disadvantageous here that two pressure sensors are necessary. As a further alternative it is disclosed that the atmospheric pressure upstream of the air filter is to be calculated from the air mass flow rate, air temperature and intake manifold pressure measured variables when the internal combustion engine is in a predefined operating state. The atmospheric pressure which is calculated in this way is then to be used in turn to determine the degree of contamination of the air filter by means of formation of pressure differences. The way in which the atmospheric pressure is to be calculated is not disclosed.
However, the problem is that, for the calculation of the atmospheric pressure, the contamination of the air filter is an important input variable which should not be neglected under any circumstances. However, according to the prior art said input variable is only calculated in a second step, from the previously calculated atmospheric pressure.
An aspect of the invention is therefore to provide a method with which both the atmospheric pressure and the degree of contamination of an air filter can be calculated, on the basis of the measured pressure in the intake manifold of an internal combustion engine, reliably and with sufficient precision.
This aspect may be achieved by determining a standardized air mass flow rate from measured values for the air mass flow rate and for the intake pressure; measuring the intake pressure with a first air mass flow rate and a second standardized air mass flow rate and calculating a pressure difference therefrom; determining a degree of contamination of the air filter from the calculated pressure difference by reference to a characteristic curve stored as a function of the pressure difference; reading out a pressure loss from a pressure difference characteristic diagram which is stored as a function of the standardized air mass flow rate and the degree of contamination of the air filter, and determining the atmospheric pressure from a sum of the intake pressure measured in the intake line and the pressure loss occurring at the air filter.
The method according to certain preferred embodiments of the invention makes it possible to determine the atmospheric pressure on the basis of the intake pressure, the intake air temperature and the air mass flow rate so that a separate atmospheric pressure sensor can be dispensed with. This is advantageous with respect to the costs and the required installation space in the intake tract of the internal combustion engine.
The problem that the degree of contamination of the air filter is not to be neglected when determining the atmospheric pressure is avoided by separating the calculation of the degree of contamination of the air filter from the calculation of the atmospheric pressure. The contamination of the air filter is calculated first without requiring the current atmospheric pressure to do so. The degree of contamination of the air is then used in the second step to calculate the atmospheric pressure.
This is made possible by standardizing the air mass flow rate to a predefined reference temperature and a predefined reference pressure. This standardization ensures that a change in pressure difference at the air filter which is caused by an increase in altitude or a change in air temperature is converted to the standardized conditions during development. By virtue of this standardization, the pressure difference then depends only on the standardized air mass flow rate and on the degree of contamination of the air filter.
By including the intake air temperature in the calculation of the standardized air mass flow rate, the precision of the method can be improved.
Depending on the operation of the engine, it is also possible that one of the two standardized air mass flow rates at which the measurements are performed does not occur over a relatively long time period. As a result, the vehicle may travel through a relatively large difference in altitude between the sensing of the respective atmospheric pressures. In this case, the method would determine an incorrect degree of contamination of the air filter. In order to prevent this, the atmospheric pressures can either be monitored directly or else it is also possible to monitor that a predefined time period or a predefined distance is not exceeded between the measurements at the two standardized air mass flow rates.
In the non-steady-state operating mode of the internal combustion engine it is possible for a phase shift to occur between the standardized air mass flow rate and the intake pressure, which leads to an error in the calculation of the atmospheric pressure. In order to prevent this, the change in the standardized air mass flow rate over time can be continuously monitored and the determination of the atmospheric pressure can be suspended during the non-steady-state operation.
As both the atmospheric pressure and the degree of contamination of the air filter are very slowly changing variables, it is possible for a first-order time delay filter for filtering out relatively small interference to be respectively provided at the output of the evaluation unit for the atmospheric pressure or for the degree of contamination of the air filter.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.